1. FIELD OF THE INVENTION
The present invention relates generally to a method of evaluating deformations of channel boxes of fuel assemblies for use in nuclear reactors and, more particularly, to a method of evaluating deformations of the channel boxes in a single exposure cycle period of a nuclear reactor. The present invention also relates to an apparatus for evaluating deformations of channel boxes of fuel assemblies for use in nuclear reactors.
2. DESCRIPTION OF THE RELATED ART
In general, fuel assemblies for use in BWR's (boiling-water type light water reactors) have a structure in which a fuel bundle including sixty-two fuel rods and two water rods is accommodated in a channel box made of zircalloy-4, the fuel rods and the water rods being arrayed in an 8.times.8 grid-like configuration and being retained by means of upper and lower tie-plates and spacers.
A multiplicity of fuel assemblies which are constructed in the above-described manner are arrayed as shown in FIG. 19 in a nuclear core. The number of fuel assemblies in the core is on the order of 400 to 764, although it depends upon the required power of each nuclear reactor.
In the core, the fuel assemblies are combined in groups each consisting of four fuel assemblies, and a cross-shape control rod is inserted into a cross-shaped gap which is formed by the adjacent corner portions of the four fuel assemblies in the middle of each group. Each fuel assembly in the core is provided with an identification number and core-array position coordinates (i, j) for identification purposes.
Deformations of the channel boxes of the aforesaid fuel assemblies in an exposure cycle period of the nuclear reactor are explained with reference to Figs. 23a and 23b.
FIGS. 23a and 23b are views which serve to illustrate the status of deformation of a particular channel box which has been used for exposure in a core. FIG. 24 is a cross-sectional view showing a single cell unit consisting of four fuel-assembly channel boxes and a cross-shaped control rod.
When BWR fuel assemblies are loaded in a reactor, a small amount of reactor coolant flows not only in a primary flow passage in each channel box but in gaps between the fuel assemblies, and a differential pressure .DELTA.P occurs between the inside and the outside of the channel box. The channel box is deformed by cree due to this differential pressure .DELTA.P and reactor-water temperatures (280-300.degree. C.) and bulges in the radial direction as shown in FIG. 23a. The amount of bulge also depends on fast neutron flux .phi. since the rate of creep of zircalloy under exposure conditions is accelerated compared to the rate of creep under non-exposure conditions. The differential pressure .DELTA.P between the inside and the outside of the channel box which forms a primary cause of the bulge reaches the maximum at the lower end of the fuel assembly, while the fast neutron flux .phi. reaches the maximum in the middle of the core in the axial direction thereof. Accordingly, the distribution of the bulge of the channel box appears in the axial direction of the core, and becomes the largest at a position about 1/3 higher than the lower end of the fuel assembly. Moreover, the bulge of the channel box depends on the loading position thereof in a core, and the bulge increases toward the middle of the core and it is the smallest at the outermost circumferential portion of the core.
The crystal grains in zircalloy which is a typical material for channel boxes, are oriented in a certain direction during production. Accordingly, in an exposure environment, zircalloy is deformed in a particular direction even in the absence of external stress. This phenomenon is referred to as irradiation growth, which forms a cause of axial bending of the entire channel box.
The amount of irradiation growth of zircalloy greatly depends on differences in material properties and exposure conditions, particularly fast neutron flux. If a great difference occurs in the fast neutron flux between opposing side surfaces of a channel box, a difference in the amount of irradiation growth occurs between these side surfaces. As a result of the difference in the amount of irradiation growth, a channel box may be deformed in the middle portion thereof as shown in FIG. 23b since the upper and lower ends of the channel box is supported on a fuel assembly. Bowing of each channel box depends on the loading position thereof in the core. In the middle portion of the core, the degree of bowing is small since the gradient of the fast neutron flux is small. However, in the outer peripheral region of the core, since fast neutron flux of a large gradient appears between opposing side surfaces of a channel box, the degree of bowing becomes large.
If the loading period of the channel box in the reactor is increased, the aforesaid bulge and bow also increase. As a result, it is likely that the gap between the channel box and an associated control rod decreases and a control rod becomes unable to be inserted in the gap between channel boxes in each group.
The gap between each channel box and an associated control rod averages about 3 mm. The amount of deformation induced by exposure greatly depends on the loading position in the core.
Accordingly, in order to increase the loading period of channel boxes over the entire region of the core, it is necessary to examine that no interference has occurred between the channel boxes and associated control rods by estimating deformations of the individual channel boxes at the respective positions in the core.
Conventionally, estimated deformations of channel boxes during the loading period thereof in a reactor have been obtained by using empirical formulae which are based on data obtained by measurement in an actually operating reactor. However, since such empirical formulae are based on data obtained in the existing exposure range, if the loading period of the channel boxes in the reactor is made long, the degree of exposure of the channel boxes greatly exceeds the degree of exposure to which the conventional empirical formulae are applicable. Accordingly, the estimated deformations of the channel boxes which have been obtained simply by extrapolating the conventional empirical formulae are excessively rough, and it is therefore necessary to make large a design margin and a safety margin in the method of using the channel box.
Conventional channel boxes are disassembled and disposed of in a nuclear reactor site, commonly after they have been used over three or four cycles. However, the production cost of channel boxes is the next highest to that of fuel rods of all the constituent elements of fuel assemblies and, in addition, disposal of the channel boxes incurs significant costs and expenses. Moreover, as waste channel boxes are accumulated, the problem that an even wider space for safety storing the waste channel boxes must be attained will be encountered.
Japanese Pat. application No. 56-59807 proposes an arrangement in which the service life of a channel is extended by altering the wall thickness of the channel box.
However, such alternation in the wall thickness of the channel box involves the problem of design change of channel boxes, with the result that the economical efficiency and the usability of channel boxes are impaired.